This invention relates to the field of organic olymeric coatings and, more particularly, to formulations and methods for applying a flexible, high temperature, thermally stable and color stable coating to a substrate such as a polyimide insulative cover for a metal conductor.
Polyimide materials such as that sold under the trade designation "Kapton" by E. I. DuPont have both favorable dielectric properties and high temperature thermal stability which render them uniquely suited for use as insulative materials for electrical conductors. Because of their resistance to oxidation and their retention of flexibility, even after long-term exposure to elevated temperatures, the polyimides have found particular application as conductor insulative material in high performance services such as, for example, aircraft and spacecraft.
Typically, metal conductors are provided with an insulative cover of polyimide by helically wrapping a polyimide tape around the conductor, with successive wraps of the tape partially overlapping each other. Commercially available polyimide tape is commonly provided with fluorinated ethylene propylene resin (FEP) backing on one or both sides. After the conductor has been wrapped with the tape, the resulting assemblage is heated to a temperature at which the FEP fuses to bond together the overlapping wraps of the tape.
Both stranded cable and single conductor wire insulated with polyimide as described above have proven to be highly serviceable in high temperature oxidative environments which are relatively destructive of other forms of insulation. Because of the deep amber color of polyimide films, however, the utility of this type of insulation is somewhat limited where there is a need for an opacified or pigmented insulation, in particular where color coding of the wire insulation is essential. For example, it has been found to be very difficult to obtain a good opaque white color by addition of white pigment to the film since the amount of pigment necessary to overcome the intense amber of the film adversely affects other properties of the insulated wire, rendering it incapable of meeting government specifications for high performance applications such as in aircraft and spacecraft. The alternative approach of utilizing a topcoating to color the insulated wire has also met with little success since most topcoating materials adversely affect the properties of the underlying polyimide or, themselves, are not capable of maintaining their stability and flexibility under the severe service conditions in which polyimide insulated conductors are used.
To be suitable for color coding of a polyimide insulation, a topcoating material must be initially flexible and resistant to embrittlement when exposed to high temperatures over long periods of time. Further, of course, the composition must retain color stability when exposed to such adverse environments. Additionally, the coating material must be adherent to the substrate and capable of masking the intense amber color of the underlying polyimide so as to provide and maintain a distinctive coloration, preferably of a pastel hue.